For example, conventional high frequency heating appliances for cooking are very convenient cooking appliances capable of heating efficiently and rapidly because only the object to be heated, e.g. food, is heated by induction. Or, depending on the menu of cooking, it is also possible to heat for a long time at low output by controlling the high frequency output for thawing, egg dishes, or long and slow cooking food such as stew, and high frequency heating appliances with an output selector have been conventionally used and provide expected effects. In particular, since stewing requires a very long heating time at low output, the heating time setting is naturally very long. For instance, while a setting of 15 minutes or 20 minutes may be sufficient for the usual high frequency output induction heating at about 500 W or 600 W, a setting of about an hour is necessary for stewing because of heating at a high frequency output of about 1/3 of the usual heating, and it is very inconvenient if the conventional appliance permits a time setting of only 15 or 20 minutes.
In one of the conventional examples, as shown in FIG. 1, the object e.g. food, to be heated (not shown) is put on a turntable 2 in a heating compartment 1, the door 3 is closed, and a timer 4 for high output is set to a proper heating time according to a menu table 5 depending on the kind and size (weight) of the food, and the cooking start button 6 is pressed, and when the timer 4 expired, the cooking ends. In this arrangement, however, two timers are required, one for usual heating and the other for long-time setting for stewing.
One of such examples is shown in FIG. 2. There is a selector for high frequency output, and a timer 4 for high output and a timer 7 for low output are used. The heating time is set by the timer 4 where high output was needed, and by the timer 7 where low output is needed for stewing or the like.
Its circuit is shown in FIG. 3, in which a time switch 8 is turned on when the low output timer 7 is actuated, and the timer motor 9 for low output begins to rotate at the same time. In this operation, the time switch 8 remains closed until expiration.
As an attempt to solve this problem, a two-speed timer 10 has been used for setting both long time and short time. That is, as shown in FIG. 4, the time setting is divided at about 20 minutes, and a heating time of up to 20 minutes can be easily set on large graduations, and a longer time is set on small timer graduations which are operating time display graduations, so that the timer operating speed may be varied by an output selector button 13 in order to set a long time.
In the heating appliance for cooking having such design, however, when the operating speed of the timer motor is varied, a time lag occurs structurally, and a discrepancy of about two or five minutes occurs relative to the setting graduation due to the error between the angle of inducator 11 of the time switch for varying the operating speed and graduations 12 of two-speed timer 10, which results in poor finishing of the cooking due to the discrepancy of heating time as described above.
Yet, since the structure is extremely complicated as compared with that of one-speed timers, and the cost of parts is as high as for two timers and the quality is inferior because of the complicated structure. The only merit is saving of space in designing.